HDU 1142 A Walk Through the Forest (记忆化搜索 最短路)

A Walk Through the Forest

Time Limit: 2000/1000 MS (Java/Others)    Memory Limit: 65536/32768 K (Java/Others)
Total Submission(s): 4383    Accepted Submission(s): 1573

Problem Description

Jimmy experiences a lot of stress at work these days, especially since his accident made working difficult. To relax after a hard day, he likes to walk home. To make things even nicer, his office is on one side of a forest, and his house is on the other. A nice walk through the forest, seeing the birds and chipmunks is quite enjoyable. 
The forest is beautiful, and Jimmy wants to take a different route everyday. He also wants to get home before dark, so he always takes a path to make progress towards his house. He considers taking a path from A to B to be progress if there exists a route from B to his home that is shorter than any possible route from A. Calculate how many different routes through the forest Jimmy might take. 

 

Input

Input contains several test cases followed by a line containing 0. Jimmy has numbered each intersection or joining of paths starting with 1. His office is numbered 1, and his house is numbered 2. The first line of each test case gives the number of intersections N, 1 < N ≤ 1000, and the number of paths M. The following M lines each contain a pair of intersections a b and an integer distance 1 ≤ d ≤ 1000000 indicating a path of length d between intersection a and a different intersection b. Jimmy may walk a path any direction he chooses. There is at most one path between any pair of intersections. 

 

Output

For each test case, output a single integer indicating the number of different routes through the forest. You may assume that this number does not exceed 2147483647

 

Sample Input

5 6 1 3 2 1 4 2 3 4 3 1 5 12 4 2 34 5 2 24 7 8 1 3 1 1 4 1 3 7 1 7 4 1 7 5 1 6 7 1 5 2 1 6 2 1 0

 

Sample Output

2

4

 

解题思路：这题与HDU的《校园漫步》差不多，也是先将每点到终点的最短距离保存起来，然后记忆化搜索！

 

解题代码：

// File Name: A Walk Through the Forest 1142.cpp
// Author: sheng
// Created Time: 2013年07月18日 星期四 19时57分53秒

#include <stdio.h>
#include <string.h>
#include <iostream>
using namespace std;

#ifdef WINDOWS
    #define LL __int64
    #define LLD "%I64d"
#else
    #define LL long long
    #define LLD "%lld"
#endif

const int max_n = 1004;
const LL INF = 0x3fffffff;
int n, m;
struct point
{
    int x, y;
};

int map[max_n][max_n];
LL vis[max_n], dis[max_n];
LL s[max_n];

LL DFS(int p)
{
    s[2] = 1;
    if (s[p] > 0)
        return s[p]; //记忆化
    for (int i = 1; i <= n; i ++)
    {
        if ((dis[i] < dis[p]) && map[p][i] < INF)
            s[p] += DFS(i);
            
    }
    return s[p];
}

int main ()
{
    while (~scanf ("%d", &n) && n)
    {
        scanf ("%d", &m);
        for (int i = 1; i <= n; i ++)
            for (int j = 1; j <= n; j ++)
                map[i][j] = INF;
        for (int i = 0; i < m; i++)
        {
            int a, b, c;
            scanf ("%d%d%d", &a, &b, &c);
            map[a][b] = map[b][a] = c;
        }
        memset(vis, 0, sizeof (vis));
        memset(s, 0, sizeof(s));
        for (int i = 1; i <= n; i ++)
        {
            dis[i] = map[2][i];
        }
        vis[2] = 1;
        for (int i = 0; i < n; i ++)
        {
            int k;
            LL min = INF;
            for (int j = 1; j <= n; j ++)
            {
                if (!vis[j] && min > dis[j])
                {
                    min = dis[j];
                    k = j;
                }
            }
            vis[k] = 1;
            for (int j = 1; j <= n; j ++)
            {
                if (!vis[j] && dis[j] > dis[k] + map[k][j])
                    dis[j] = dis[k] + map[k][j];
            }
        }
        dis[2] = 0; //因为从2到2是距离为0的
        DFS(1);
        printf (LLD"
", s[1]);
    }
    return 0;
}